I'm 20 weeks pregnant with our third baby, last night I woke up in the middle of the night with soaking wet underwear and pyjama pants. What would be the cause of this, it's never happened to me before?

The most common cause is that without realizing it you lost control of your bladder, and had temporary incontinence (for example you might have sneezed) causing some urine leakage. However, if you are continuing to lose clear fluid then you should get checked out by your doctor straight away as it is possible that you have had a leak of of amniotic fluid from the membranes.